112 



BJ0RN HELLAND-HANSEN 



[REP. OF THE "MICHAEL SARS" NORTH 



58 is due to variation of the tides, a current with a diur- 

 nal period and a maximum velocity of at least 9 cm./sec. 

 must be superposed on the one with a semi-diurnal period. 



The observations here discussed leave room for no 

 doubt as to the existence of fairly strong tidal currents 

 in the upper water-layers in the open sea at a consider- 

 able distance from the Azores. The depth to the bottom 

 was about 950 metres at Stat. 58, as mentioned above. 

 The Station was situated in a locality where the bottom 

 sloped from the Azoric sub-marine platform towards the 

 deep sea in the south. It is to be expected that the velo- 

 city of the tidal currents should increase in such a locality, 

 but it is nevertheless remarkable that they are as important 

 as our observations show. 



The measurements made at depths greater than 9 

 metres are illustrated in the graph on p. 102'-. They are 

 too sparse to allow of a satisfactory analysis. The varia- 

 tions in the observed velocity and direction of the current 

 at different levels do not agree with the variations in the 

 heading of the ship, and the movements of the latter have 

 not had a decisive influence upon the results of the measure- 

 ments so far as one can judge from the scattered observations. 

 In Fig. 46 the results for 46, 91, 183, 457 and 732 metres 

 are illustrated by means of vectors. At each of these depths 

 4 observations were made. It seems fairly sure that the 

 current at all levels turned cum sole, and that a complete 

 turn took place in the course of about 12 hours. This 

 evidently means that the rotatory tidal currents dominated 

 at all depths. They were not equally strong or running 

 in the same direction at the different levels. 



Such variations of the tidal currents from one level 

 to another support the conclusions arrived at in Chapter 

 V. We shall not go further into the subject here. A more 

 thorough discussion of the interesting problems that arise 

 must be left for the future when a great deal more material 

 in the form of observations has been procured. 



46. Measurements from Drifting Ship. 



The current measurements made at Stats. 19 C, 47 and 

 49 C will not be discussed in detail. They only serve to 

 demonstrate a few facts of interest with regard to the 

 variations according to depth. 



The observations at Stat. 19 C were made when the 

 ship was moving slowly eastwards (with the trawl at (he 

 bottom). The station was situated in the Mediterranean 

 at a short distance from Gibraltar. 



The current-meter at 300 metres registered a velocity 

 of about 60 cm./sec. and a direction contrary to that in 

 which the ship was drifting. It is probable that the cur- 

 rent-meter here acted almost like a log. At 5 metres the 



20 



1 JUNt 2 Juiit t910 



22 25 -^ Z 



W 



80 



n 

 bo 



50 



to 



iO 

 20 



to 

 N 



!0 

 ZO 

 50 



w 



50 

 bO 

 70 

 80 



5 



/"V\A 



\ 



5TAT.^9C 



^W''^^i 



274 



* 1829 



91 *" A57 



/' Vfc ^ AA r 



+ 1629 • 9,,, 



\.,.' 



N 



Fig. 47. Relative velocities and directions observed at Stat. 



E 



49 C, 



deviation of the compass of the instrument makes the 

 observations unreliable, but it seems fairly certain that the 

 apparent current ran eastwards at a considerable velocity 

 (about 60 cm./sec). This means that the surface current 

 had a real velocity of more than 1 metre per second 

 towards E. at the time of observation, which corresponds 

 to the time when the surface current in the Straits of 

 Gibraltar had passed its maximum but still ran strongly 

 eastwards. At 20 metres the current was deflected to the 

 right in relation to the current at 5 metres, and at 40 

 metres it was still more deflected. At both of these depths 

 the current moved eastwards at a greater speed than the 

 ship. At 80 metres the apparent current ran towards SW 

 at 1'' 16™, the real current probably running SE at a speed 

 considerably less than at higher levels. At 3'' 4"' the indi- 

 cation of the current-meter at 80 metres seems nearly to 

 correspond to the movement of the ship, as is also the 

 case with the observation at 170 metres. 



At Stat. 47 a few measurements were made when the 

 trawl was hauled and the ship moving slowly. The direc- 

 tions registered by the current-meter were nearly opposite 

 to the heading of the ship, the greatest difference (26°) 



